There are a number of industrial processes, such as coal gasification, biomass gasification, steam reforming of hydrocarbons, partial oxidation of natural gas, etc., which produce gas streams that include CO2, H2 and CO. It is frequently desirable to remove CO2 from those gas mixtures to capture CO2, for example for sequestration purposes and to produce H2 or H2-enriched gas product.
One process commonly used in the industry today involves amine-based gas scrubbers. In these scrubbers, the gas mixture is contacted with an amine-containing organic solvent or an amine-containing solution. CO2 and other acidic molecules, such as H2S, are selectively absorbed in the amine solution. Once the solution is saturated with CO2, it is regenerated to release CO2 gas molecules, and the solvent is recycled to the absorption step. Such processes can have significant drawbacks. For example, they can use large amounts of amine solvents; they can require continuous absorption/regeneration cycles; and, generally speaking, they are capital and energy-intensive processes. For at least these reasons, membrane technologies, which are known to be more energy and capital efficient than the conventional solvent-based separation processes, have been long sought.
Membranes made of polymeric materials have been developed and commercially used for molecular separation, such as separating CO2 from natural gas streams. However, polymeric membranes are associated with poor thermal and chemical stability, and their permeation flux is often low. Moreover, hydrocarbons ubiquitously exist in CO2 gas mixtures derived from fossil fuel sources, and these hydrocarbons can cause degradation of the polymeric membranes by dissolution, fouling, etc., further limiting widespread use of polymeric membranes.
Inorganic membranes are an emerging technology area and hold high promise to overcome the thermal and chemical stability issues that are associated with polymeric membrane materials. Among the inorganic membrane materials studied so far, zeolite or molecular sieve membranes are considered to be most promising, because zeolite materials have been used as catalysts and/or adsorbents in the industry for many decades and offer molecular-level lattice channel structures to discriminate individual molecules based on their slight difference in weight, size, and/or shape. However, CO2 separation functions of inorganic membranes have not been well demonstrated yet, perhaps because making a defect-free inorganic membrane in a practical way remains a large material processing challenge. In addition, conventional inorganic membranes frequently offer much lower surface area packing density than do polymeric membranes because of the inorganic membrane's tubular or planar disk forms, as illustrated in FIGS. 1A and 1B. In FIGS. 1A and 1B, arrow 902 represents a gas mixture that is to be separated; arrow 904 represents permeate; and arrow 906 represents retentate. The conventional inorganic membrane technology can also impose a large manufacturing and engineering cost based on the unit-membrane-separation-area, further limiting widespread application of zeolite and other inorganic membranes.
In view of the forgoing, there is a need for materials and methods that can be used for molecular level separations, and the present invention is directed, at least in part, to addressing this need.